The routine feature size of microchips has dramatically declined to .about.0.1 .mu.m. Although a further decrease is likely, once the line size on integrated circuits becomes &lt;0.01 .mu.m, several quantum limitations will likely curtail the proper performance of solid state devices which use electron currents as signal representations. New technological schemes will need to be developed for use at these small dimensions.
Molecular scale electronics is a field of study that proposes the use of single molecules to function as the key components in future computational devices. In particular, single molecules that have strategically placed charge barriers could serve as switches and logic devices.
Typical molecular-sized systems would permit the use of .about.10.sup.13 logic gates per cm.sup.2 compared to the present feature of a microchip, less than 10.sup.8 gates per cm.sup.2, thereby offering a 10.sup.5 decrease in required size dimensions. In addition to substantial size reductions, the response times of molecular devices can be in the range of femto-seconds while the fastest present devices operate in the nanosecond regime. Thus a 10.sup.6 increase in speed may be attainable. Although numerous obstacles remain, a combined 10.sup.11 increase in computing performance offers an exciting impetus to consider molecular scale electronic architectures for future ultracomputing.
Present electronic digital devices are governed by considerations of size and speed. Optimizing the size of the basic units (usually the transistors) and their speed (limited by their natural temporal responses) are conflicting design goals. Therefore several trade-offs have to be made. The most important compromise in computational technology is the hardware-software duality, which materializes in the requirements of a programmed logic (memory- or software-dominant) versus wired logic (CPU-, or hardware-dominant). Components of programmed logic are smaller and able to handle larger problems than a wired logic system; however, a wired-logic is faster than a programmed-logic. At one extreme there can be a bit adder (a minimum logic unit able to sum) with a small number of logical gates that will require a large memory to obtain the results, while at the other extreme, there could be a large CPU with all specific functions wired into the system that will be able to process the entire problem, having only a small memory for the input and output data. Present technology is heavily inclined toward programmed logic, for example, a computer with a large memory and a fast but simple CPU. This requires very large programs to solve present problems.
A classical two-terminal semiconductor device is characterized by the ability to conduct charges (electrons or holes) with low impedance in one direction and high impedance in the reverse direction. The simplest of these two-terminal devices is a diode where a p-type semiconductor is joined to an n-type semiconductor (p-n junction). Therefore, an electrical potential applied from one direction will deplete the junction and will not allow a current to pass, while a potential applied in the opposite direction will allow the crossing of charged carriers. A more advanced device is a transistor where an additional collector, p- or n-type semiconductor, is joined to the n- or p- side of the diode, or base, respectively. The main feature of this device is that a small current in the base is able to control a large current in the collector or emitter. This notable amplification can also be viewed as a device having a low input impedance and a high output impedance. A parallel device which can be made even smaller than the bipolar transistor is the field effect transistor (FET). It is similar to the transistor but the output current is controlled by an input voltage rather than an input current. It has a three terminal arrangement: gate, source and drain corresponding to the base, emitter and collector, respectively, of the bipolar transistor.
These three-terminal electronic devices can be classified as amplifiers and switches. Normally good amplifiers are also good switches, but the opposite is not necessarily true. When the transistor is used as an amplifier, the goal is to increase the power (energy per unit of time) of an input signal. If the device has a small input resistance, it will have a small input power, and if it has a large output resistance, it will have a large output power. The power of the signal is amplified at the expense of a power supply, thus an amplifier shapes the energy of a power supply according to the shape of a small input signal. Since the output must sustain a large amount of energy under amplification conditions, the use of single molecules as amplifiers is discouraged and is therefore not being considered within the paradigm described here. However, the other application of transistors is their use as digital systems for processing of information, such as in digital computers. This is the area where single molecules can be used with possibilities of making computers orders of magnitude more efficient.
In bulk electronics, it is a simple task to convert a good amplifier to a switch in which a small signal controls the passage of current; therefore the logic is based on whether or not the current passed. A large output current provides the circuit with a capable fan out, namely the ability to drive subsequent circuits in a cascade with high reliability. Logical circuits, in addition to performing the logical operations, also need to have a good driving capability. The output of a gate needs to be powerful enough to excite the input of one or more subsequent gates. At the same time, this high output should not couple to the input of the same device. This implies that there must be a very high impedance between the input and output. These are problems that would limit the use of molecular devices if bulk device architectural philosophies are applied. The energy of a power supply could not be shaped using a single molecule.
The eventual limitation to downscaling of conventional devices will be brought about by a combination of difficulties in fabrication, reliability, yield, interconnects, economics of production, and ultimately fundamental limits. In this size regime, several fundamental properties of quantum systems have to be considered such as superposition, interference, entanglement, non-clonability and uncertainty. Therefore, extension of microelectronics beyond current size regimes requires exploration of non-conventional electronic structures which scale far beyond these limits.
Since conventional devices would be difficult to downsize, novel approaches to logic circuit design have to be devised wherein transistors and integrated circuits are substituted by using simple molecular structures. Of course, this implies the consideration of non-standard architectures and design philosophies. Entirely new logic architectures would be required from molecular based electronic components which will be complementary yet non-identical to their bulk counterparts. Hence, molecules that work like the basic electronic devices (transistors, diodes etc.) may not be desired, except for purely testing purposes. However, as we consider novel designs, we should not discard some basic principles that underpin CPU construction. For example, proposals in molecular scale electronics must regard the need for input/output signal homogeneity within devices. An electronic input and a photonic output (or vise versa) within a device would be difficult to consider since the second device in the series would then require photonic operation. We must maintain a homogeneity of input and output signal types (e.g. voltage in and voltage out) and magnitudes within a device so that the second device can be driven with the same signal type and signal size that operated the first device. Therefore, new architectural strategies must be proposed while considering fundamental needs of overall CPU operation.
If devices were to be based upon single molecules, using routine chemical syntheses, one could prepare 6.times.10.sup.23 (Avogadro's number) devices in a single reaction flask, hence, more devices than are presently in use by all the computational systems combined, world-wide. Thus, molecular scale electronics would likely shift the software/hardware equilibrium in the direction of hardware, namely, massively wired logic or CPU dominant. Molecular circuits offer the possibility of constructing large and fast CPUs with complicated functions. Using large molecular arrays, problems could be calculated within the CPU with minimal need for main or auxiliary memories. Wired-logic would supplant much of the programmed-logic, thereby affording several additional orders of magnitude increase in performance.
A major consideration in molecular devices is the energy consumption/dissipation needs. Transfer of large numbers of electrons or electron currents would lead to excessive heat problems with molecular scale devices, and such a strategy may only be useful for testing purposes. Considering 10.sup.8 gates/cm.sup.2 (as in presently-sized silicon-based systems) at the rate of 10.sup.-9 sec (present speeds) yields 10.sup.17 electrons/sec (.about.0.02 amperes/cm.sup.2) if only one electron per gate is used to transport, indicate, fetch, or represent a binary digit. At this point heat considerations are already extreme: if the average resistance of the circuit is 30.OMEGA., this represents 20 watts/cm.sup.2. If an increase of several orders of magnitude in performance is expected with molecular circuits, this would imply a proportional increase in power dissipation. Such levels of power dissipation rule out most conventional current or electron transfer methods for practical molecular devices wherein large numbers of devices are densely configured.
In spite of these and other hurdles, the enormity of Avogadro's number, the richness of molecular structures available, and the physical advantages of using systems in a quantized regime make the prospects of molecular scale electronics eminently attractive for the next generation of ultracomputing. Thus, there is a keen interest and need for digital electronic circuit elements at the molecular scale.